1. Field of the Invention
The present invention relates to an apparatus and method for detecting very small distances, and more particularly to proximity sensing.
2. Background Information
Many automated manufacturing processes require the sensing of the distance between a manufacturing tool and the product or material surface being worked. In some situations, such as semiconductor lithography, the distance must be measured with accuracy approaching a nanometer.
The challenges associated with creating a proximity sensor of such accuracy are significant, particularly in the context of photolithography systems. In the photolithography context, in addition to being non-intrusive and having the ability to precisely detect very small distances, the proximity sensor can not introduce contaminants or come in contact with the work surface, typically a semiconductor wafer. Occurrence of either situation may significantly degrade or ruin the semiconductor quality.
Different types of proximity sensors are available to measure very small distances. Examples of proximity sensors include capacitance and optical gauges. These proximity sensors have serious shortcomings when used in photolithography systems because physical properties of materials deposited on wafers may impact the precision of these devices. For example, capacitance gauges, being dependent on the concentration of electric charges, can yield spurious proximity readings in locations where one type of material (e.g., metal) is concentrated. Another class of problems occurs when exotic wafers made of non-conductive and/or photosensitive materials, such as Gallium Arsenide (GaAs) and Indium Phosphide (InP), are used. In these cases, capacitance and optical gauges may provide spurious results.
U.S. Pat. No. 4,953,388, entitled Air Gauge Sensor, issued Sep. 4, 1990 to Andrew Barada (“'388 patent”), and U.S. Pat. No. 4,550,592, entitled Pneumatic Gauging Circuit, issued Nov. 5, 1985 to Michel Deschape (“'592 patent”), disclose an alternative approach to proximity sensing that uses an air gauge sensor. U.S. Pat. Nos. 4,953,388 and 4,550,592 are incorporated herein by reference in their entireties. These sensors use reference and measurement nozzles to emit an air flow onto reference and measurement surfaces and measure back pressure differences within the sensors to measure the distance between the measurement nozzle and the measurement surface.
Furthermore, principles of pneumatic gauging are discussed in Burrows, V. R., The Principles and Applications of Pneumatic Gauging, FWP Journal, October 1976, pp. 31–42, which is incorporated herein by reference in its entirety. An air gauge sensor is not vulnerable to concentrations of electric charges or electrical, optical and other physical properties of a wafer surface. Current semiconductor manufacturing, however, requires that proximity is gauged with high precision on the order of nanometers. Earlier versions of air gauge sensors, however, often do not meet today's lithography requirements for precision.
As indicated above proximity sensors must be non-intrusive. Contact between a proximity sensor and a work surface can significantly degrade or run the semiconductor quality of quality of other work surface. However, to ensure the greatest level of precision often the measurement nozzle must be extremely close to the work surface. In certain circumstances, as higher levels of precision are required, the movement of a wafer stage or other work platform is such that it is desirable to move a proximity sensor toward and away from a work surface. This leads to another source of imprecision related to the mechanical stability of a proximity sensor head, which is move up and down. When the sensor head is extended it can drift thereby reducing the accuracy of the proximity sensor.
What is needed is a retractable proximity sensor that includes a self compensating mechanism to reduce the impact of proximity sensor head drift on the accuracy of the proximity sensor.